I'm 15 and if I were you I would be a bit more strategic than you are being. It's obvious you're starting to obsess about her a little so you need to cool that down a little bit. Limit checking your facebook, twitter etc to twice per day and don't search her. Make that a rule. Secondly find things to do which don't involve her, even if they are solo things or better still join clubs where she isn't going to be rpesent. After a few weeks re-evaluate how you feel, so you haven't just wound yourself up into a crush on someone, which is easy to do.
I have someone I am interested in who I also consider out of my league at the moment for other reasons, I'm very clinical about it and I have a 3 year plan about it without it becoming the centre of my world.If it happens great, if it doesn't, there are other things to be doing. I don't think your weight will matter if she likes you as a potential boyfriend but Jim is right, she's been your platonic friend for a while, maybe she would have taken it forward if she was interested, but if she's a party girl maybe she just doesn't want a boyfriend full stop. Anyone. Not just you. I think if you give yourself space and time things will mellow out as they are supposed to, don't rush anything and don't be crushed if she isn't interested at this point. All things are potentially changeable in time, and there are always other people as well.xx